v0.3.0

Added

• servicrab exec <SERVICE> -- <COMMAND>..., for the questions that start with
  "but what does the service actually see?":

  servicrab exec api -- printenv DATABASE_URL
  servicrab exec api -- npm run migrate
  servicrab exec db -- psql

  It runs the command with the service's merged environment, its env_file
  layers and its working directory, assembled by the same code that starts the
  service, so the two cannot drift apart. The command inherits servicrab's
  stdio — interactive tools and pipes work, and nothing of servicrab's own
  reaches the output — and its exit status is passed through, with 127 for a
  command that does not exist and 126 for one that is not executable, as a
  shell would.

  Unlike docker exec it does not enter a running process: no daemon is
  involved and no namespace is joined. That is the point. Debugging a service
  that refuses to start is exactly when there is nothing to attach to, and it is
  also the limit — a variable the process changed after startup is not visible
  here.

  Everything after the service name belongs to the command, including its own
  flags.